When is a nova not a nova? When a white dwarf and a brown dwarf collide

Researchers from Keele University and an international team of astronomers reported for the first time that a white dwarf and a brown dwarf clashed in a "torch of glory" that was observed on Earth in 1670.

In Chile, Chile, astronomers have discovered evidence that a white dwarf (the remnants of a sun-like star at the end of his life) and a brown dwarf (a failed star without sufficient mass to maintain the thermonuclear fusion) in an ephemeral explosion of glory that was seen on Earth in 1670 as Nova Cygni – "a new star under the head of the Swan". It suddenly appeared like a star as bright as those of the plow, which gradually faded, reappeared and finally disappeared.

Modern astronomers who studied the remains of this cosmic event originally thought that it had been caused by the fusion of two stars of the main sequence on the same evolutionary trajectory as our sun. This nova was for a long time called "Nova Vulpeculae 1670", and later became known as CK Vulpeculae. However, we now know that CK Vulpeculae was not what we would call today a nova, but rather the fusion of two stars: a white dwarf and a brown dwarf.

By studying the debris of this explosion, which now appears in the form of two rings of dust and hourglass-like gas with a compact central object, the research team concluded the merger between a brown dwarf and a white dwarf. Professor Nye Evans, Professor of Astrophysics at Keele University and co-author of Monthly Notices from the Royal Astronomical Societyexplains: "CK Vulpeculae has been considered in the past as the oldest" old nova. "However, the observations of CK Vulpeculae that I have made over the years at the ground telescope and in space m & # 39; convinced that it was not a nova Everyone knew what it was not, but no one knew what it was With our observation by ALMA of the exquisite dusty hourglass and the curled disc , as well as the presence of lithium and abundant isotopes particular, the puzzle nestles: in 1670, a brown dwarf star is shredded and thrown to the surface a white dwarf star leading to eruption from 1670 and to the hourglass that we see today. "

The team of European, American and South African astronomers used Aracama Large Millimeter / submillimeter Array to examine the remains of the fusion and reported some interesting results. By studying the light of two more distant stars while they were shining through the dusty remains of the fusion, the researchers were able to detect the revealing signature of the lithium element, which is easily destroyed in stellar interiors.

Stewart Eyres, vice-dean of the Faculty of Computer Science, Engineering and Science at the University of South Wales and lead author of the paper, states: "The material contained in the hourglass contains the The lithium element, normally easily destroyed in stellar interiors, the presence of lithium, as well as the unusual isotopic ratios of the elements C, N, O, indicate that a very small amount of material, in the form of lithium, is present. a brown dwarf star, crushed on the surface of a white dwarf in 1670, leading to burning thermonucleation, an eruption that led to the lightening observed by Carthusian monk Anthelme and the astronomer Hevelius, and in the hourglass we see today. "

Professor Albert Zijlstra, co-author of the study, School of Physics and Astronomy at the University of Manchester, said: "Stellar collisions are the most important events. The focus is on collisions between neutron stars, or between two dwarfs – which can produce a supernova – and planet-star collisions, but it is very rare to see a collision, and where we believe one, it's hard to know what kind of star collided in. The collision is new, never seen before or seen before, it's an extremely exciting discovery. "

Professor Sumner Starrfield, professor of astrophysics at Arizona State University, said: "The white dwarf would have been about 10 times more massive than the brown dwarf, so the brown dwarf exploded in the white dwarf, he would have The tidal forces exerted by the white dwarf caused, when they collided, a cocktail of unusual molecules and isotopes, organic molecules that we could detect with ALMA have spread measurably in the surrounding environment C & # 39; is the first time that such an event is conclusively identified.Unfortunately, the hourglass is also rich in organic molecules such as formaldehyde (H₂CO), methanol (CH₃OH) and methanamide (NH₂CHO). These molecules would not survive in a nuclear fusion environment and must have been produced in the debris of the explosion. This further confirms the finding that a brown dwarf collapsed in a star-to-star collision with a white dwarf. "

Since most star systems in the Milky Way are binary, stellar collisions are not that rare, astronomers note. Professor Starrfield said, "Such collisions are probably not uncommon and this material will eventually become part of a new planetary system, implying that they may already contain the building blocks of organic molecules underway. training. "

Explore further:
Binary white dwarf stars

More information:
Eyres et al., ALMA, unveil the consequences of a merger between a white dwarf and a brown dwarf at CK Vulpeculae, Monthly Notices from the Royal Astronomical Society (2018). DOI: 10.1093 / mnras / sty2554